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As its a new year I thought it was time to flaunt my ignorance a bit and request others to explain something that confuses me.
Many deep sky observers use small aperture refractors because they have a nice short focal length, generate no diffraction spikes around the bright stars and are easily luggable.
The trend seems to be for these to have objectives consisting of 3, 4 or even 5 lenses (often using expensive ED glasses to keep all the wavelengths cofocal on the images). This inevitably leads to more expensive telescopes because more glass has to be worked and they are correspondingly more expensive to design and construct. So, we often see ads in magazines where a 75mm-100mm aperture scope sells for £2k-£4k – and thats more than you would pay for an 8″ RASA with buckets more light grasp. So, not really the cheap option.
As many observers are principally interested in narrow band filtering it begs the question, why not buy a cheap achromat with a generic field flattener and use narrow band filters for oxygen, hydrogen and sulphur separately to create 3 images and then combine them using the info in the WCS header for each image (resampling could handle any plate scale differences and orientation changes).
It looks to me like, if you avoid the most expensive filters, you could probably save £1k on the deal.
Is it the convenience of using OSCs and a single filter that dominate this decision?
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